Joe Biden is chairman of the powerful Senate Foreign Relations Committee, and has represented the small Eastern state of Delaware in the U.S. Senate for 36 years. The inclusion of the 65-year-old Biden on the ticket could help fight Republican arguments that Obama lacks experience in foreign policy.

The Obama campaign announced the decision early Saturday morning in a text message to supporters, after word had leaked out. A day earlier, Obama spoke on CBS television's The Early Show about the qualities he wanted in a running mate.

"Obviously, the most important question is, is this person prepared to be president? The second most important question from my perspective is, can this person govern?" he said.

Obama also said he wanted a vice president who will be independent.

"I want somebody who is going to be able to challenge my thinking and not simply be a 'yes person' when it comes to policy," he said.

Obama, at age 47, is a relative newcomer to national politics, with less than four years in the Senate. His Republican opponent, veteran Arizona Senator John McCain, has argued that Obama lacks experience in foreign policy and national security matters.

Biden has led the Senate Foreign Relations and Judiciary Committees, and is a vocal critic of the Bush administration's foreign policy. He voted in 2002 to authorize the U.S.-led invasion of Iraq, but later became a critic of the war.

The Delaware Senator is a Roman Catholic and son of a car salesman who could help attract working-class votes, a weak area for Obama.

Biden ran for president twice, in 1988, and this year, both times dropping out early in the campaign.

By choosing Biden, Obama passed over several other potential running mates, including former first lady Hillary Clinton, his main rival for the Democratic presidential nomination. Other finalists included Virginia Governor Tim Kaine, Kansas Governor Kathleen Sebelius, Indiana Senator Evan Bayh and Texas Congressman Chet Edwards.

Obama and Biden are scheduled to make their first joint appearance Saturday, at 1900 UTC, at a rally in Obama's home state of Illinois. They will be formally nominated at the Democratic National Convention, which opens Monday in Denver, Colorado. Obama will give his acceptance speech next Thursday, before an expected crowd of 75,000 at a football stadium.

Republican Party officials say McCain has not decided on a running mate. Former Massachusetts Governor Mitt Romney and Minnesota Governor Tim Pawlenty are said to be under serious consideration.

The Republican convention begins September 1 in Saint Paul, Minnesota.